Plant breeders have long been attempting to increase productivity of the more important crops used for food, or for processing as feed, fiber and pharmaceuticals, by their efforts in developing cultivars (cultivated varieties) with particularly desirable characteristics. One of the ways in which this aim is frequently accomplished is the development of superior plant lines by infusing desirable traits with already existing cultivars, thus potentially forming a hybrid with exceptional characteristics. The general superiority of F.sub.1 hybrids over either of their parents is a widespread phenomenon in a variety of different types of crops. This superiority may express itself in such features as increased height, growth rate, leaf area, early flowering and overall higher yields.
One way in which the production of superior plant lines has been achieved in the past is by the making of numerous manual cross pollinations to obtain the desired F.sub.1 hybrid. These crosses generally are carried out between an already existing cultivated crop variety and an unadapted or "wild type" gene donor which possesses one or more traits which the breeder wishes to incorporate into the cultivated variety. Once the production of the F.sub.1 has been accomplished, repeated backcrossings and selections are then required to ultimately obtain a plant containing all the characteristics of the cultivated plant as well as retaining the new, desirable traits introduced from the "wild type" plants. As can easily be seen, this selection procedure is extremely tedious and time consuming; yet, in spite of the difficulties, it remains one of the most widespread of plant breeding techniques currently in use.
Because of the problems involved with this method, a number of other avenues for more efficient production of F.sub.1 hybrids have been and are being explored. Among the most avidly pursued fields of endeavor is the construction of male sterile lines within the varieties of crop plants to which hybridization is desired. The principle behind the development of male sterile lines is that, in order to produce hybrid seed more economically, the restrictions of controlled cross-fertilization imposed by floral morphology, especially of perfect flowers, must be overcome. To this end, the female parent should be prevented from self- or intraline fertilization. The elimination of self-fertilization requires andro-self sterility, or the inability of the plant to produce viable pollen. The establishment of the male sterile line thus renders any crop variety readily adaptable to hybridization with virtually any gene donor having the desired characteristics, and eliminates the need for laborious hand pollination.
Male sterile lines may be established in a number of ways. Hand emasculation is one method by which a line may be sterilized. For example, large scale production of hybrid corn may be done by detasselling the female parent; however, the large scale emasculation of species having perfect flowers generally proves to be economically unfeasible.
Genetic male sterility is also a known trait, usually inherited as a recessive and monogenic trait in a number of different types of plants. Exploitation of this characteristic is used to produce hybrid seed of barley, tomato, pepper, marigold, zinnia, and others. However, there is a basic shortcoming in the use of this technique, in that it is difficult to obtain a 100% genetic male sterile stand. Overcoming this difficulty requires a rather complex use of clever genetic manipulation. Its use, therefore is currently restricted to hybrid seed production of cultivated plants in which cytoplasmic male sterility has not been found, or that in which the male sterile plasmatype exhibits inferior agronomic performance.
Cytoplasmic male sterility provides an additional mechanism for providing the desired lines for use in hybridization. In this situation, the genetic factors controlling male sterility are found in the cytoplasm. This trait is probably associated with some alteration of the normal structure or function of mitochondria of plastids. Cytoplasmic male sterility has found widespread application in the production of hybrid seed. Widespread production based on this trait is responsible for larger percentages of many important cultivated crops such as sorghum, sugarbeet, onions, melon, and, most successfully, corn. A number of difficulties exist with this system as well. First, it is difficult to ensure the expression of cytoplasmic male sterility across the range of environments in which hybrid seed may be produced. A female wheat plant which is 100% sterile in one locality may prove to be only 50% sterile in another locality, thus producing obvious difficulties in hybrid seed production.
Furthermore, once the sterile line is established, the female line must be maintained through the use of a male fertile maintainer line; and hybrid seed must be restored to at least semi-sterility via a "restorer" line. Clearly the necessary development of effective and appropriate maintainer and restorer lines presents a considerable obstacle to the efficient and economical exploitation of the trait for the production of hybrid seed. In fact, a number of important cereal crops, such as wheat, have continued to resist all efforts to establish efficient cytoplasmic male sterility restorer lines.
A method of producing male sterile lines which circumvents the difficulties of genetic induction is the use of chemical sterilization agents. The principle involved here is that the chemical acts as a gametocide selectively altering the male gamete, i.e., pollen, by inducing physiological abnormalities, which in turn prevent pollen development, pollen shed, or pollen viability. A number of chemical compounds have been shown to have at least a partial effect in producing male sterility in plants. Among these are: 2-chloroethylphosphonic acid (ethephon; Berhe et al., Crop Science 18: 35-38, 1978); sodium 1-(p-chlorophenyl)-1,2-dihydro-4,6-dimethyl-2-oxonicotinate (RH-531+532; Jan et al., Euphytica 23: 78-85, 1974); 3-(p-chlorophenyl)-6-methoxy-s-triazine-2,4 (1H,3H) dione-triethanolamine (DPX3778; Johnson et al., Crop Science 18: 1026-1028; 2,7-diamino-10-ethyl-6-phenylantridium bromide (ethidium bromide; Burton et al., Crop Science 16: 731-2, 1976). Although use of these compounds obviate the problems encountered with genetic manipulation, there are still a number of difficulties which might arise with use of chemisterilants. For example, chemical treatment may result in induction of only partial sterility, or may be variable in the degree of male sterility induced under field conditions. They also may produce phytotoxic side effects, such as seed shrinking, which may reduce the viability and/or agricultural utility. Further, female sterility may also be induced by the use of some of these chemicals. Another undesirable feature is that for the most part, these compounds are applied as foliar sprays. The necessity of such application presents the problem of environmental pollution, and synchrony with the exact plant stage of development.
It has now been discovered that another class of compounds, DABCO (1,4-Diazabicyclo[2.2.2]octane) and its quaternary salt derivatives have the unexpected effect of causing male sterility in plants. The subject compounds, which are preferably used to treat seeds directly, are thus not limited to use as foliar chemisterilant sprays, although this is an alternate method of application. Use of these compounds for chemical emasculation allows for the development of all female plants which may be used to produce large quantities of hybrid seed. These compounds have the added advantage of producing male sterility in wheat, a plant which has traditionally resisted all attempts to establish a successful hybrid seed production program.